FIGS. 1 and 2 illustrate a typical nuclear steam generator 1. Such nuclear steam generators are formed with a primary side 3 and a secondary side 5 which are hydraulically isolated from each other by a tube sheet 7. The primary side is generally of a bowl-shaped configuration that is subdivided by a divider plate 9 into two halves that are sealed against direct flow from one half to the other. An inlet half 10 (known as an inlet channel head) receives radioactive water that has been circulated through a nuclear reactor via a water inlet 11, and an outlet half 12 (known as an outlet channel head) discharges water from the steam generator 1 back to the nuclear reactor via a water outlet 13, as represented by the arrows in FIG. 2. Between the inlet and outlet halves 10, 12 of the primary side 3, the hot radioactive water is circulated through a heat exchanger 15 of the primary side formed from a bundle of U-shaped heat exchanger tubes 16 that are located within the secondary side 5.
The bundle of U-shaped heat exchanger tubes 16 will typically have approximately 3500 tubes, each of which has a hot leg 17, a cold leg 19 and a U-shaped bend 21 connecting them. Open bottom ends of the hot legs 17 and the cold legs 19 are secured within openings in the tube sheet 7 in a leak-proof manner, so that the open ends of the hot legs 17 communicate with the inlet channel head 10 and the open ends of the cold legs communicate with the outlet channel head. Thus, a U-shaped flow path for the radioactive water through the heat exchanger 15 established.
Within the secondary side, the bundle of heat exchanger tubes 16 are uniformly positioned within a plurality of axially spaced support plates 25. Some of the support plates can be fixed to a central divider plate 27 and to a wrapper 29 that is disposed between the bundle of tubes 16 and the outer shell 31 of the steam generator 1. Conventionally, vertical support for the support plates is provided by a plurality of stay rods and spacer pipes (not shown). To receive the legs 17, 19 of the heat exchanger tubes, each of these support plates 25 is provided with tube openings 33. These openings 33 have a diameter that is slightly larger than the outer diameter of the heat exchanger tubes extending therethrough in order to facilitate assembly. Thus, once assembled, a tube-to-plate clearance gap 35 will exist.
Nonradioactive water is delivered to the cold side of the secondary side 5 via a feed nozzle 36 and a preheater distribution box 37. The nonradioactive water is circulated vertically within the heat exchanger 15 in any of a number of ways. Where axial flow preheating is provided, the plates 25 can be an open-work structure that freely allows a flow of water through them. On the other hand, when cross-flow type preheating is utilized, the plates 25 can be low leakage baffles with flow windows, such as that represented at 38 in FIG. 3B.
At the top of the secondary side 5 of the steam generator 1, a steam drying assembly 39 (FIG. 1) is provided for extracting water from the wet steam that is produced by boiling of the nonradioactive water within the heat exchanger 15. This steam drying assembly 39 includes a primary separator 41 and a secondary separator 43. Dry steam rising above the separator assembly 39 is conducted to a steam turbine (not shown) for driving an electrical generator (also not shown). Water extracted from the steam passing through the steam drying assembly 39 is directed into a downcomer path between the wrapper 29 and the shell 31, through which it can travel down to the bottom of the secondary side 5.
As already mentioned, flow of nonradioactive water within the heat exchanger 15 is vertically oriented. However, whether axial preheating or cross flow preheating is provided, cross flows can act upon the cold legs 19 of the heat exchanger tubes in at least the zone containing the preheater distribution box. Because of the clearance gap 35 existing between the cold legs 19 and the tube openings 33 in the support plates 25, in any areas where significant cross flows exist, undesirable tube vibration and wear can occur. Furthermore, if the zone within which cross flows is created are increased to increase heat exchanger efficiency, this problem will be further exacerbated.
Thus, there is a need for a method and heat exchanger which eliminates wear-producing vibrations between the heat exchanger tubes and the support plate openings without eliminating the oversizing of the support plate tube openings relative to the outer diameter of the heat exchanger tubes which serves to facilitate assembly of the heat exchanger.